Additive manufacturing has become a more and more attractive solution for the manufacturing of metallic functional prototypes and components. It is known that SLM, SLS and EBM methods use powder material as base material. The component or article is generated directly from a powder bed. Other additive manufacturing methods, such laser metal forming (LMF), laser engineered net shape (LENS) or direct metal deposition (DMD) locally fuse material onto an existing part. This newly generated material may be deposited either as wire or as powder, where the powder deposition device is moved along a predefined path with either a robot or a CNC machine. Build on machined preforms with SLM is known prior art for die-casting, where stainless steel or other known die-cast alloys are used.
FIG. 1 shows a basic SLM arrangement 10, known from the prior art, wherein a three-dimensional article (component) 11 is manufactured by successive addition of powder layers 12 of a predetermined layer thickness d, area and contour, which are then melted by means of a scanned laser beam 14 from a laser device 13 and controlled by a control unit 15.
Usually, the scan vectors of one layer are parallel to each other within that layer (see FIG. 2a) or defined areas (so called chest board patterns) have a fixed angle between the scan vectors in one layer (see FIG. 3a). Between subsequent layers (that means between layer n and layer n+1; and between layer n+1 and layer n+2 and so on) the scan vectors are either rotated by an angle of for example 90° (see FIG. 2b, 3b) or by an angle different of 90° or n*90°, (see FIG. 4a, 4b). This (using alternating scanner paths for subsequent layers or for certain areas of a pattern, e.g. chest board, within one layer of the article) was done so far to achieve a good quality (optimum part/article density and geometrical accuracy) with respect to an article made by SLM.
A typical SLM track alignment known from the state of the art is shown in FIG. 5.
Due to the typical temperature profile in the melt pool and the resulting thermal gradients in the vicinity of the melt pool, a faster and preferred grain growth perpendicular to the powder plane (x-y plane) is favoured. This results in a characteristic microstructure showing elongated grains in the z-direction (=primary grain orientation direction, crystallographic [001] direction). This direction is perpendicular to the x-y plane. Therefore, a first specimen extending in z-direction (see FIG. 1) shows properties different from a second specimen extending in the x-y plane (=secondary grain orientation direction, secondary crystallographic direction), for example the Young's modulus along the z-direction is generally different than the Young's modulus in the powder plane (x-y plane).
Therefore, one characteristic feature of powder-based or other additive manufacturing technology is the strong anisotropy of material properties (for example Young's modulus, yield strength, tensile strength, low cycle fatigue behaviour, creep) resulting from the known layer-wise build-up process and the local solidification conditions during the SLM powder bed processing.
Such anisotropy of material properties could be a disadvantage in several applications. Therefore, the applicant has already filed two so far unpublished patent applications, which disclose that the anisotropic material behaviour of components manufactured by additive laser manufacturing techniques can be reduced by an appropriate “post-built” heat treatment, resulting in more isotropic material properties.
During the last 3 decades directionally solidified (DS) and single-crystal (SX) turbine components were developed, which are produced by investment casting and where low values of for example the Young's modulus in primary and secondary grain orientation (normal to the primary growth direction) are aligned with thermo-mechanical load conditions. Such an alignment is here provided by application of seed crystals and grain selectors and has resulted in a significant increase of the components performance and lifetime.
However, so far such techniques to control the primary as well as the secondary crystallographic orientation were not known for parts/components produced by SLM.
It has also become possible to control the microstructure of deposits formed on single-crystal (SX) substrates with generative laser processes, a technique called epitaxial laser metal forming (E-LMF). These methods can produce parts, which have either a preferred grain orientation (DS—directionally solidified) or an absence of grain boundaries (SX—single crystal).
With increasing design complexity of future hot gas path components, the economic manufacturing of such SX or DS parts/components by casting will become more and more problematic, as the casting yield for thin- or double walled components is expected to drop. Moreover, epitaxial laser metal forming can be only applied to parts, where the base material has already a single crystal orientation.
The SLM technique is able to manufacture high performance and complex shaped parts due to its capability to generate very sophisticated designs directly from a powder bed.
A similar control of the microstructure as described above for cast SX or DS components would be thus highly beneficial for parts and prototypes which are manufactured with the SLM technique or other additive manufacturing laser techniques. An additional control and alignment of the Young's modulus would further increase the performance and application potential of such components.
Therefore the applicant has already filed a so far unpublished patent application, which discloses a method for manufacturing a metallic component/a three-dimensional article by additive manufacturing, preferably by Selective Laser Melting (SLM), with improved properties of the component, where the anisotropic properties of the component could either be used in a favourable manner, or where anisotropy could be reduced or avoided, depending on the design intent for the component. Furthermore, an appropriate method was described for realizing an alignment of the anisotropic properties of the article with the local thermo-mechanical load conditions. A controlled secondary grain orientation is realized by applying a specific scanning pattern of the energy beam.
Unfortunately, additive manufacturing, for example SLM, has a low build rate as a disadvantage. This is an obstacle for its commercial success because of the long process times with associated high manufacturing costs and low to moderate throughput for serial production.